The synthesis of organic compounds, as in general many chemical processes, is nowadays subjected to demanding requirements of environment compliance.
A continuous effort is made in order to implement chemical processes which have a minimum environmental impact and this field of research is known as “green chemistry”.
However, the synthesis must satisfy industrial requirements, especially good yields, final purity of the product and plant management.
Aromatic aldehydes are an important category of chemicals which are used for example in pharmaceuticals, cosmetics, food industry, agrochemicals, dyes and plastic additives.
A convenient synthetic route to obtain aromatic aldehydes is represented by the oxidation of the corresponding alcohols or alkenylbenzenes.
In the mainstream of the “green chemistry”, photocatalytic oxidation of an alcohol is a convenient way to obtain aldehydes.
Generally, photocatalytic oxidation is used in detoxification of water from organic pollutants (Amat et al., Applied catalysis B: Environmental 73 (2007) 220-226).
Palmisano et al., Adv. Synth. Catal. 2007, 349, 964-970, report the photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde in water with organic-free suspensions of titanium dioxide. The only by-products present were traces of 4-methoxybenzoic acid and aliphatic products, carbon dioxide being the other main oxidation product. The work of Palmisano et coll. aims at investigating the nature of the TiO2 photocatalyst and at their best, the selectivity of p-anisaldehyde is 41.5%.
Augugliaro et al., Photochemical & Photobiological Sciences, 2009, 8, 663-669, and Yurdakal et al., Green Chem. 2009, 11, 510-516, illustrate photocatalytic partial oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde in organic-free water suspensions of TiO2 catalysts of different phases with selectivities to p-anisaldehyde up to about 74%. As already observed in other works, the Authors explain that oxidation of the alcohol to the corresponding aldehyde is partly affected by the direct mineralization of the alcohol to CO2 and by overoxidation of the aldehyde to open ring derivatives.
This kind of problem generally can occur with the preparation of aromatic aldehydes by means of oxidation reactions.
There is the evident need to achieve a more accurate control of an oxidation reaction in order to avoid side products which lower the yield of the final product and make more difficult to obtain it in a pure form.
Higashimoto et al., Journal of Catalysis 266 (2009) 279-285, achieve yield of benzaldehyde higher than 95% from benzyl alcohol by carefully selecting the type of TiO2 catalyst. It must be observed, however that the process is carried out in acetonitrile. This solvent is to be abandoned in view of safety and environmental impact.
Other kinds of TiO2 catalysts are disclosed in CN101531575.
However, the problem to achieve an optimal control of the oxidation reaction and good separation of the final product from the reaction media still remains.
Many purification and separation methods are well known to the person of ordinary skill in the art. For example liquid mixtures can be separated by adsorption or distillation, but these methods are affected by high costs, especially in terms of equipment, energy and safety. Moreover, in this specific context of heterogeneous photocatalysis, wherein the catalyst is in an extremely fine powdery form, there is the need to improve separation methods of the liquid product from the catalyst.
Other problems derive from the intrinsic difficulties of operating in a continuous mode.
Pervaporation is a well-known method used for separating liquid mixtures. A liquid mixture is contacted with a non-porous membrane. The compounds in the liquid mixture permeate the membrane and then evaporate downstream. Different solubilities and diffusivities of each compound make the separation possible. Flow is maintained by applying a vacuum downstream or with a carrier gas.
Use of pervaporation is well documented in the art. Membrane technology is disclosed for example in JP58089901, EP0311882, EP0381477. These membranes are used to separate water from alcohols or ketones, but there is no disclosure of separation of an aldehyde from its original reactant.
EP0423949 and EP0584414 disclose non-porous separating membranes used in a pervaporation process to separate an aliphatic alcohol with less than three carbon atoms from oxygenated organic compounds, such as ethers, aldehydes, ketones and esters. Alcohol permeates the membrane, while oxygenated compounds remain in the retentate.
EP1167333 discloses a pervaporation process for removing water from the reaction media in the preparation of acetals and ketals.
Boddeker et al. Journal of Membrane Science 137 (1997) 155-158 disclose the isolation of vanillin from bioconversion broth by means of pervaporation.
WO2009/130245 discloses the use of pervaporation process to partially separate water from a reaction process comprising condensation, dehydration and hydrogenation of ketones and aldehydes.
DE4337231 discloses a process wherein the concentration of formaldehyde is obtained by pervaporation of water through a particular membrane, which is impermeable to the aldehyde.
The problem of separating the aldehyde from its corresponding alcohol, or in general from its corresponding starting compound in an oxidation reaction is not disclosed in the above mentioned references. This problem has been faced, but not solved in satisfactory manner.
An attempt to control oxidation reaction of an alcohol to the homologous aldehyde and at the same time separating the final product is disclosed in Benguergoura et al., Journal of Membrane Science 229 (2004) 107-116. The Authors make essential that oxidation reaction is carried out in anhydrous environment for its control, which is contrary to the purpose of the present invention, which is directed to reactions in aqueous, preferably organic-free media.
Pervaporation is used to separate alcohol from its corresponding aldehyde and this is enabled for n-propanol/n-propanaldehyde and n-butanol/n-butyraldehyde, but the Authors stress out a poor selectivity between alcohol and its corresponding aldehyde. In fact, pervaporation through conventional polydimethylsiloxane (PDMS) is found effective for preventing further oxidation of primary alcohols, however, the overall yield of the reaction, in terms of aldehyde is affected by the simultaneous permeation of the alcohol. The Authors propose to overcome this problem by developing a suitably chemically-designed membrane. Also, the oxidation system is a solution K2CrO7/H2SO4, which is completely different from the photo-oxidation system used in the present invention.
Solovieva et al., Journal of Membrane Science 110 (1996), 253-255 disclose a catalytic process of alcohol oxidation with separation of the final product by pervaporation. However, the experimental results are not very clear. It must be observed that the oxidation reaction is not controlled satisfactorily, since the acids are always formed. The separation mechanism (ion-exchange) is different from the solution-diffusion mechanism usually operating in pervaporation. Furthermore the results are not clear. The membrane seems to be used mainly with the aims to act as a “carrier” for the catalyst (see cited reference 2: “Polymer sulfofluoride films as carriers for metalloporphyrin catalysts”) and to obtain a gaseous stream with the aldehydes. It appears obvious that the aldheydes must be largely diluted in the permeate carrier gas, otherwise they cannot permeate and need a further separation stage. From FIG. 2 one cannot understand if the aldheydes are formed at the feed side or inside the polymer, and it is not said which is the fate of the residual reagents and of the side-products. The only clear result is that the “effective oxidation rate constant” is 100 times lower than the one obtained in heterogeneous photocatalysis: “The value is less than k in the case of heterogeneous catalysis by a factor of 10−2.”. On the basis of these results any expert designer would discard this method to utilize the heterogeneous photocatalysis with a possible post reaction separation.
The prior art provides information only for short-chain linear aliphatic alcohol/aldehyde series. That information cannot be transferred to aromatic series.
Kanani et al., Ind. Chem. Res. 2003, 42, 6924-6932 study recovery of tea aroma components by pervaporation technique. This work reports separation factors for binary and multicomponent mixtures. Although separation factors are acceptable in tea extract, when measured in binary or multicomponent situation, they are not very good. Moreover, Kanani et al. show the behaviour of phenylacetaldehyde and benzyl alcohol, which is not a corresponding couple, hence cannot be used as predictable model for the oxidation reaction. The skilled person would not find any successful indication.
She et al., Journal of Membrane Science, 271 (2006) 16-28 dissert on a theoretical basis about the separation factors in pervaporation of flavour organics. They conclude that interaction effects among different components are usually not relevant and remain substantially the same both in a binary (water/compound) and a multicomponent mixture. Also in this case, there are no data on a corresponding alcohol/aldehyde couple, so that this paper cannot be used to make any prediction.
Camera-Roda and Santarelli, Journal of Solar Energy Engineering, February 2007, 129, 68-73 discuss intensification of water detoxification by integrating photocatalysis and pervaporation. As far it is known to the present inventors, this is the only piece of prior art combining photocatalytic oxidation and pervaporation. However, this work aims to a different goal than the one of the present invention. Actually, the reaction oxidation of an aromatic alcohol is enhanced as much as possible to mineralization, namely to CO2, and no control of the reaction is envisaged.
The mechanism also is different. In fact photocatalysis is utilized to transform a poorly permeating compound (4-chlorophenol) into other much more permeable intermediate compounds. So, in this “detoxification”, with aims that are clearly very different from those of a chemical synthesis, intermediate compounds pervaporate faster than the original pollutant and concurrently it is not necessary that photocatalysis complete their mineralization.
Therefore, the state of the art does not provide any guidance on how to achieve the accurate control of the oxidation of an aromatic alcohol to the corresponding aldehyde in a photocatalytic non-organic, aqueous environment.